Fuel grade ethanol distilled from grain has become increasingly popular as an alternate fuel for motor vehicles. Ethanol has also increased in popularity as a gasoline additive for formulating clean burning grades of gasoline for motor vehicles.
A fuel grade ethanol production process typically includes the steps of fermentation, distillation and evaporation. In the fermentation portion of a typical process, a slurry of milled corn is fermented to produce a beer having a concentration of ethanol that is usually no more than approximately 15% by volume. In the distillation portion of a typical process, the ethanol in the beer is extracted in distillation columns. Distillation columns typically have a multitude of horizontal trays for bringing rising ethanol vapor and descending liquid into contact. In a distillation column, low pressure steam percolates up through the beer as the beer cascades from higher trays to lower trays. As the rising steam heats the beer, the ethanol in the beer evaporates and rises to the top of the column where it exits as a vapor. The remaining water and other grain material in the beer descends to the bottom of the column to exit as “beer bottoms”. After solids have been removed from the beer bottoms, the remaining liquid known as thin stillage is reduced in the evaporation portion of the process where liquid is boiled away from the thin stillage to produce a syrup.
To produce fuel grade ethanol, more than one interconnected distillation column is typically used to progressively purify the ethanol product. In a typical ethanol distillation process, a beer column receives beer and produces an intermediate ethanol vapor. A rectifier column receives the intermediate ethanol vapor from the beer column and produces 190 proof or 95% pure ethanol vapor. A third, side stripper column receives bottoms from the rectifier column and then produces an intermediate ethanol overhead vapor that is further purified by the rectifier column. The ethanol free bottoms from the side stripper column can be used to formulate cook water for the fermentation portion of the process. Because of the physical properties of an ethanol water solution, a distillation process can only practically produce an ethanol water solution that is approximately 95% ethanol and 5% water. A dehydrator is used to remove most of the remaining water to produce higher purity product. The dehydrator receives the 95% ethanol vapor and removes nearly all of the remaining water to produce ethanol having a water content of less than 0.25%. A dehydrator may contain beads of material which attract water to a greater degree than ethanol.
A fuel grade ethanol distillation process like the one described above also produces co-products. The fermentation portion of the process generates carbon dioxide gas which is often recovered for various industrial uses. The distillation portion of the process generates the above mentioned beer bottom byproduct that is free of ethanol and which contains unfermented solid remnants of the milled grain that was fermented to produce ethanol. This beer bottom byproduct can be mechanically separated into a mostly liquid component known as thin stillage and a mostly solid component know as distiller's grains. A drier can be employed to dry the distiller's grains to produce dry product. The distiller's grains are high in protein and therefore make an excellent feed for farm livestock. Because releasing the thin stillage would amount to a release of waste water, the thin stillage is usually evaporated in evaporators to produce a syrup which can also be dried in the distiller's grain dryer to further increase the output of the animal feed co-product.
The economic constraints confronting a fuel grade ethanol producer are more challenging than those faced by a distiller of spirits for human consumption. This is because fuel grade ethanol must have virtually no water content and it must be produced at low cost. Accordingly, it is an object of this invention to provide a process arrangement for distilling ethanol that conserves energy and water. It is another object of this invention to provide a process arrangement for distilling ethanol that uses sets of evaporators that can each be taken off-line for maintenance while the evaporation portion of the process continues to operate at full capacity. It is yet another object of this invention to provide an ethanol distillation process arrangement that uses waste heat from the evaporators to heat the distillation portion of the process.